Fisher
optical encoder

ABSTRACT

OPTICAL SHAFT ENCODER FOR DIGITALLY INDICATING SHAFT POSITION. ENCODER HAS CODE WHEEL WITH A PLURALITY OF CIRCULAR BINARY CODE ZONES AND A PLURALITY OF READER UNITS, ONE FOR EACH ZONE FOR RECEIVING LIGHT TRANSMITTED THROUGH THE WHEEL TO READ THE ZONES. A MASK IS POSITIONED BETWEEN THE LIGHT SOURCE AND READER UNITS AND HAS SLITS, ONE FOR EACH ZONE, THROUGH WHICH THE LIGHT PASSES. THE SLITS INCREASE IN CIRCUMFERENTIAL EXTENT PROCEEDING OUTWARDLY OF THE WHEEL AND HAVE SIDES WHICH LIE ALONG THE RADII OF TWO CODE WHEEL AXIS. A SINGLE CODE ZONE IS READ TO OBTAIN TWO DIGITS OF A NUMBER REPRESENTING THE POSITION OF THE   WHEEL BY USING PHASE DISPLACED READERS. THE SLITS ARE SUCH TO PROVIDE EQUAL AMOUNTS OF LIGHT AT EACH READER.

Feb. 16,1971 Re. 27,063

OPTICAL ENGODER Original Filed Sept. 16, 1960 4 Shets-Sheet 1 AR In II a I I 2 N S) N 8 Q a i N\ J. y j Q 4 N fi 5;- INVENTQR, N Jamv 55/ /59 A r- TOR/Y5K? Feb. 16, 1971 FISHER Re. 27,063

OPTICAL ENCODER OriginalFiled Se'pt. 15. 1960 4 Shets-Sheet n uu gfi ommnnu nunuuuWL Z INVENTOR.

' JOHN fish H? Fbb.l6,1971 J.HSHER Re.2L068 Y OPTICAL ENCODER Original Filed Sent. 16, 1960 Y 4 Sheets-Sheet S E3 'UVVEN7DR.

Johw HsHER Feb. 16, 1971 J, H HER Re. 27,063

OPT ICAL ENCODER Original Filed Sent. 16, 1960 4 Sheets-Sheet 4 INVENTOR- Jo/w fish ER A T TOR/VfYS United States Patent OPTICAL ENCODER John Fisher, Aurora, Ohio, assignor to The Warner & gvfiasey Company, Cleveland, Ohio, a corporation of [0 Original No. 3,388,392, dated June 11, 1968, Ser. No. 393,807, Aug. 24, 1964, which is a continuation of abandoned application Ser. No. 56,590, Sept. 16, 1960. Application for reissue June 30, 1969, Ser. No. 847,478

Int. Cl. G08c 9/06 U.S. Cl. 340-347 16 Claims Matter enclosed in heavy brackets appears in the original patent but forms no part of this reissue specification; matter printed in italics indicates the additions made by reissue.

ABSTRACT OF THE DISCLOSURE Optical shaft encoder for digitally indicating shaft position. Encoder has code wheel with a plurality of circular binary code zones and a plurality of reader units, one for each zone for receiving light transmitted through the wheel to read the zones. A mask is positioned between the light source and reader units and has slits, one for each zone, through which the light passes. The slits increase in circumferential extent proceeding outwardly of the wheel and have sides which lie along the radii of the code wheel axis. A single code zone is read to obtain two digits of a number representing the position of the wheel by using phase displaced readers. The slits are such to provide equal amounts of light at each reader.

The present invention relates to encoders and particularly to optical encoders for digitally indicating shaft position.

This application is a continuation of my copending application Ser. No. 56,590, filed on Sept. 16, 1960, now abandoned.

An important object of the present invention is to provide a new and an improved encoder for digitally indicating shaft position, which encoder is highly accurate and relatively free from ambiguity but, yet, is compact in structure so as to accommodate its use in locations where space is at a premium.

A further object of the present invention is to provide a new and an improved optical encoder for converting shaft position into a number comprised of a plurality of binary digits, the encoder including a code wheel having a plurality of radially displaced zones extending circularly about the code wheel with each zone being comprised of alternate transparent and opaque areas and a light-responsive reader unit for reading each of the zones with the encoder being so constructed and arranged that the electrical outputs of the reader units for each zone are substantially the same when the units are subjected to maximum illumination for the corresponding zone.

It is also an object of the present invention to provide a code wheel for indicating shaft position in the Gray, or reflected binary, code with the code wheel having a digit value code zone for each digit of the Gray number except for the two digits of the Gray code which are representable by the two coarsest digit zones and which have the same number of divisions but which are angularly displaced from each other, these digits being represented by a single zone read by a reader which is angularly displaced.

In the above objects and in the following description and claims the word transparent area is intended to cover an area which transmits light and includes an open slit in an otherwise opaque wheel.

In accordance with the preferred embodiments of the present invention, the possibility of obtaining a false or Re. 27,063 Reissued Feb. 16, 1971 ICC an ambiguous reading from a code wheel having a plurality of circularly extending digit zone tracks thereon radially displaced from each other and read by lightresponsive reader units disposed on one side of the wheel and responsive to light from a light source on the other side of the wheel disposed inwardly of the zone tracks and which illuminates the reader units through the transparent areas of the code wheel as they rotate past the units is minimized by providing a mask adjacent the code wheel with the mask having slits which correspond to respective ones of the zones to control the amount of light which passes from the light source through the transparent areas to the photocells and to provide more definite resolution as the zone being read by the reader unit changes from a transparent to an opaque area. The slits for the zones have dilferent areas, with the areas of the slits being such that substantially equal quantities of light are transmitted by the slits when subject to full illumination. I have found that there is a point as one proceeds inwardly toward the light source, where the areas of the slits must be increased as one moves closer to the light source, rather than decreased as one would expect, to pass a given quantity of light. In other words, a slit of given area and defining a given arc, once inside this point, will pass less light as it is moved inwardly with the area and arc maintained constant. In the preferred embodiment, the slits are made as wide as possible in a circumferential direction with all slits subtending the same angular arc wherever possible and the radial extents of the slits for the zones varied to provide the area necessary for each slit to transmit the same amount of light as sensed by the reading units. The radial extents of the corresponding zones vary in accordance with the radial extents of the slits. Where the slits all subtend the same angular are or increase in circumferential extent as one moves outwardly, the radial extent of the slits and zones can be made to decrease. In the preferred and illustrated embodiments, the angular arc subtended by the slit of the fine zone of the wheel is about half that of the angular arc subtended by the other slits of the encoder. In addition to the foregoing, two light responsive elements and corresponding slits are preferably provided for reading each zone with one photo cell going dark as the other photocell goes light, with the outputs of the photocells being combined to provide the output signal of the reader unit for the zone. The construction as described, in addition to minimizing false and ambiguous readings, facilitates the provision of a compact wheel since the use of slits which have a greater circumferential extent as one proceeds away from the light source eliminates the need for extending the radial extent of the slits, and in turn of the zones to acquire a wheel which has reader units subjected to substantially equal amounts of light. The compactness of the wheel is further facilitated by interchanging zones of the wheel so that when proceeding outwardly from the center of the wheel, the zones do not progressively increase in divisions or resolution. In the preferred and illustrated embodiment, the wheel is coded in the Gray code where the successive digits of the code are respectively represented by zones which have 2 2 2 2 2 divisions, and the zones which would normally appear as the third and fourth digit zones of the code wheel proceeding outwardly from the center of the wheel, are interchanged so that they are positioned between the zones representing the sixth and seventh digits. Furthermore, the first and second digits are represented by a single zone with the single zone being read by two reader units angularly displaced from each other.

The elimination of false and ambiguous readings is also facilitated by supporting the light-responsive eleis important that stray light does not destroy the accuracy of the reading by the photoelectric cells. To provide improved definition, a mask is disposed between the code wheel and the enlarged part 16 and is designated by the reference numeral 50. This mask could also be positioned immediately in front of the wheel. The mask 50 has a slit opposite each of the photocells and the slits have been given the same reference character as the zones read by the corresponding photocells with an s appended thereto. It will be seen that the slits for the photocells for reading the coarsest zone 25 are designated by the reference character 25s; while those slits disposed opposite the photocells for reading the outermost zone are designated by the reference character 33s, etc. The mask 50 has an opening 51 therein through which the shaft 11 extends and is clamped in position by a clamp block 52 which clamps the mask against a cooperating block 53 fixed to the enlarged part 1-6. It will be appreciated that a plurality of such pairs of clamp blocks is provided for mounting the mask.

In accordance with the present invention, the slits are proportioned so that with full illumination from the light source the slits will pass substantially the same quantity of light. Although the intensity of the light from the light source has an inverse relationship with respect to the distance from the source, I have found that when slits are positioned close to the axis, as in the preferred embodiment, a slit of a given area will pass a quantity of light which has a direct relationship to its distance from the slit rather than an inverse relationship. Consequently, the slits 25s, 26s, 27s, 28s, 29s, actually may have to decrease in area in the order named to provide slits which transmit substantially equal quantities of light when subject to full illumination. In the encoder which was constructed, the slits 25s-29s were respectively positioned from the axis of the mask, and the areas of the slits decreased proceeding outwardly from the axis of the mask.

Preferably, for all zones where it is possible, the slits subtend the same angular are as the slits 25s so that the same angular movement of the wheel is necessary to move the edge of an opaque or transparent area from one side of the slit to the other and the area is adjusted by varying the radial extent of the slits. Referring to FIG. 3, it will be apparent that the slits decrease in radial extent proceeding outwardly from the filament to provide the area necessary for passing the same quantity of light as the slits of the more inwardly disposed zones. FIG. 3A has the slits all rotated to a position where the center line of the slits lie along a single radius. It is apparent therefrom that the radially extending sides of the slits lie along radii which in each case define equal angles with each other with the exception of the slit for the finest zone. Furthermore, the radial extent of each slit varies to provide slits having the areas necessary to provide the same quantity of light when the slits are subjected to full illumination by the light source. It will be noted that the slits 33s for the finest zone subtend an arc of a much smaller angle than the other slits and this is due to the fact that the divisions of the fine zone in the illustrated code wheel are so fine that a larger slit would interfere with the definition obtained. Accordingly, the slit is made as wide as possible consonant with the requirements of definition, and the radial extent of the slit is such as to provide the necessary slit area so that the slit passes the same quantity of light as the other slits.

It will be noted that the radial extents of the zones vary in the same manner as the slits and decrease proceeding outwardly from the center of the code wheel except for the finest zone. This arrangement enables the diameter of the wheel to be kept at a minimum. The variation in radial extent of the zones is clearly shown in FIG. 2.

As pointed out hereinbefore, the photoelectric cells for reading the various zones are not aligned on a single zero reading line but rather the zero lines of the various zones are angularly displaced from each other and the cells are correspondingly displaced so that the photocells are distributed about the code wheel. This facilitates the use of two cells for each zone in a small space without interfering with the definition of the wheel.

While the Gray code has been utilized in the described structure, it will be understood that certain aspects of the present invention are equally applicable to code wheels coded in other codes, such as the straight binary code. For example, the use of different size slits and the interchanging of zones would be useful in a binary coded wheel.

Furthermore, certain aspects of the present invention, such as the interchanging of the zones of the wheel and the use of a single code zone where the code has two digits represented by code zones with equal number of divisions are applicable in encoders using other than photoelectric means to read the code on the wheel, for example, the code could consist of areas which are alternately magnetic and nonmagnetic or electrically conductive and nonelectrically conductive.

The preferred embodiment of the present invention has been described as utilizing two photoelectric elements or cells for reading each of the zones which represent individual digits of the number representing the angular position of the shaft. The two cells for reading a zone are interconnected to provide one output level when a transparent area is opposite to one cell and a lower output level when an opaque area is opposite to the same cell. The manner of combining the output of the photocells to provide a single output signal having a high level when a transparent area is opposite to one of the photocells and a low level when an opaque area is opposite to the same photocell does not, per se, form a part of this invention and has not, therefore, been described. Suffice it to say, that for the purpose of the present invention, when two photocells are utilized for reading a zone, regardless of whether it is in the manner described or for some other reason, such as to compensate for backlash, the displacing of the zones so that the photocells are distributed about the code wheel facilitates the mechanical construction of the wheel in compact form. One encoding device where two photocells are utilized to read each zone of a code wheel is described in Patent No. 2,7 79,539 issued to Sidney Darlington.

The code wheel and disk have not been described in detail since the techniques for making code wheels hav ing alternate opaque and transparent areas are known and the same techniques may be utilized for constructing the disk of the present invention, Preferably, the code wheel is a glass code wheel having a photographic surface on one side thereof which is developed to provide the opaque and transparent areas. This technique of manufacturing a code disk is a known technique. The mask is preferably constructed in the same manner as the code disk.

The preferred and described embodiment of the present invention utilizes photoelectric cells as light-responsive elements for reading the code Wheel. It will be appreciated that other light-responsive elements may be used. Furthermore, it will be recognized that other radiation which can be either blocked or transmitted by a code wheel and mask in the same manner as light is the equivalent of light insofar as the present invention is concerned and a source of such radiation and devices responsive to such radiation would be the equivalent of the photocells and light source shown herein.

While the preferred embodiment of the present invention has been described in considerable detail, it is hereby my intention to cover all modifications, constructions, and arrangements which are apparent to those skilled in the art and which fall within the scope and spirit of the present invention.

Having described my invention, I claim:

1. A coding device for digitally indicating shaft position comprising a code wheel having a code thereon in the orm of a plurality of concentric code zones each com- )rised of alternate transparent and opaque areas, said :ones being displaced radially from each other and each IOIIG extending circularly about the axis of said wheel, a vlurality of reader units including a unit for each zone lisposed on one side of said wheel and positioned to re eive light through the transparent areas of the correponding zone as the areas rotate by the unit, said units ach including a light-responsive element, a light source mounted on the other side of said wheel and disposed adially inwardly of said wheel from said reader units, a ight blocking mask adjacent said wheel and blocking light raveling from said source to said reader units, said mask raving a slit therein opposed to each reader unit and vdjacent the corresponding zone to allow the passage if light from the light source through a transparent area f the zone when passing said slit to said light-responsive lements, said slits having areas proportioned such that he quantity of light passed by each slit when subjected to ull illumination from said source through a transparent .rea of said code being substantially the same for all of aid slits.

2. An encoding device as defined in claim 1 wherein ach of said reader units comprises a light-responsive element disposed to intercept the light transmitted through be transparent areas of the corresponding zone and the .orresponding slit, a member supporting each of said units vith the optical axes of said elements being inclined with espect to said Wheel and along the path of the light reeived by the element from said light source.

3. A coding device as defined in claim 1 wherein said lits have sides lying along radii emanating from the exended axis of said wheel with the radii for each slit deining a given angle which is the same for a plurality of aid slits for dilferent zones and wherein the radial extent at said plurality of slits and the corresponding zones dereases as one proceeds outwardly from the center of said vheel.

4. An encoding device as defined in claim 3 wherein ach of said reader units comprises a light-responsive elenent disposed to intercept the light transmitted through he transparent areas of the corresponding zone and the :orresponding slit, a member supporting each of said units vith the optical axes of said elements being inclined with espect to the wheel and lying along the path of the light eceived by the element from said light source.

5. In a coding device as defined in claim 1 wherein the adial extent of a plurality of said slits and zones decreases Iroceeding outwardly from the center of said wheel and he circumferential extent of said slits increases proceedng outwardly from the center of said wheel.

6. A code wheel as defined in claim 1 wherein said code 5 the Gray code in which two digits of the Gray numbers LI'B representable by zones having the same number of livisions but which zones are displaced angularly with espect to each other, said code wheel having a single zone hereon representing both of said digits of the code and W0 angularly displaced reader units for said single zone.

7. A coding device as defined in claim 1 wherein more han two of said code zones have different numbers of livisions therein and wherein at least one of said plurality )f code zones is disposed between zones which each have l greater number of divisions than said one of said pluralty of code zones, and said reader units each comprises tngularly displaced reading elements.

8. A code wheel as defined in claim 7 wherein said code s the Gray code in which two digits of the Gray numbers ire representable by zones having the same number of livisions but which zones are displaced angularly with espect to each other, said code wheel having a single :one thereon representing both of said digits of the code and two angularly displaced reader units for said single tone.

9. A coding device comprising a code wheel having 1 plurality of concentrically arranged circular code zones thereon with each zone comprising alternate transparent and opaque areas, a light source disposed on one side of said zones and inwardly of said wheel with respect to said zones, each of said zones having a respective light-responsive reader unit for reading [each of said zones] the zone, supported adjacent the other side of said wheel adjacent the zone read by the unit, said units each comprising a light-responsive element, a light mask mounted adjacent said wheel and having a slit corresponding to each of said zones and adapted to pass light from said source to said element of said corresponding reader unit for the zone when a transparent area of the zone is moving past the slit and reader unit, said slits increasing in circumferential extent proceeding outwardly from the center of said wheel and said zones and slits decreasing in radial extent proceeding outwardly from the center of said wheel.

10. A code wheel as defined in claim 9 wherein said code is the Gray code in which two digits of the Gray numbers are representable by zones having the same number of divisions but which Zones are displaced angularly with respect to each other, said code wheels having a single zone thereon representing both of said digits of the code and two angularly displaced reader units for said single zone.

11. A coding device comprising a code wheel having a plurality of concentrically arranged circular code zones thereon with each zone comprising alternate transparent and opaque areas, a light source disposed on one side of said zones and inwardly of said wheel with respect to said zones, a respective light-responsive reader unit for [reading each of said zones] each zone supported adjacent the other side of said wheel adjacent the zone read by the unit, a light mask mounted adjacent said wheel and having a slit corresponding to each of said zones and disposed intermediate the reader unit for the zone and the wheel and adapted to pass light transmitted by a transparent area of the zone to the reader unit, said slits increasing in circumferential extent proceeding outwardly from the center of said wheel and said zones and slits decreasing in radial extent proceeding outwardly from the center of said wheel, said slits having sides lying substantially along radii emanating from extended axis of said wheel with the angle defined by the radii for each of said slits being substantially the same as the angle defined by the radii of the other slits.

[12. A coding device comprising a member having a code thereon in the form of opaque and transparents areas, said code lying in a plane, a light source on one side of said code and positioned such that light traveling from said source through a transparent area of said code travels a path inclined with respect to said plane, a reader unit comprising a light-responsive element supported on the other side of'said member and adapted to receive light from said source when a transparent area is opposite to said element, a support member mounting said unit with the optical axis thereof along the path for light from said source through a transparent area to said unit] 13. A code Wheel as defined in claim 11 wherein said code is the Gray code in which two digits of the Gray numbers are representable by zones having the same number of divisions but which zones are displaced angularly with respect to each other, said code wheel having a single zone thereon representing both of said digits of the code and two angularly displaced reader units for said single zone.

14. A coding device for digitally indicating angular position by a number having a plurality of digits therein [and] each representable by divisions of a circular zone on a code wheel with certain digits of said number being representable by identical code zones [having the same number of divisions] but angularly displaced from one another, a code wheel having a plurality of concentric code zones thereon representing said digits and including a single code zone representing said certain digits and reader units corresponding in number to said certain digits for reading said single code zone, said units being angularly displaced from one another.

15. A coding device comprising a code wheel having a plurality of concentrically arranged circular code zones thereon with each zone comprising alternate transparent and opaque areas, a light source disposed on one side of said zones, a plurality of light responsive reader units for reading said zones including a respective light-responsive reader unit for each zone supported adjacent the other side of said wheel adjacent the zone read by the unit, each said units comprising a light-responsive element, a light mask mounted adjacent said wheel and having slits including one for each of said zones which is adapted to pass light from said source to said element of said corresponding reader unit for the zone when a transparent area of the zone is moving past the slit and reader unit, said slits increasing in circumferential extent proceeding outwardly from the center of said wheel.

16. A coding device as defined in claim 15 wherein said slits have sides extending along radii emanating from the axis of said wheel with the angle defined by the radii for References Cited The following references, cited by the Examiner, are of record in the patented file of this patent or the original patent.

UNITED STATES PATENTS 2,679,644 5/1954 Lippel et a1 340'347X 3,142,835 7/1964 Larky 340347 3,196,431 7/1965 Papelian 340347 3,218,626 11/1967 Schuman 340347 FOREIGN PATENTS 932,135 8/ 1960 Great Britain.

MAYNARD R. WILBUR, Primary Examiner G. R. EDWARDS, Assistant Examiner 

